1. Field of the Invention
The present invention relates to an image reading apparatus that quickly determines an appropriate light amount for each line period.
2. Description of the Related Art
Image reading apparatuses include a light emitting element that emits a light, a light receiving element that receives the reflected light, and a reference density board. For example, when a power supply of an image reading apparatus is switched on, the light emitting element emits a light on the reference density board, and the light receiving element receives the light reflected by the reference density board. The image reading apparatus adjusts, based on the received light amount, the light amount emitted by the light emitting element.
Because a line period differs for each image reading setting when the image reading apparatus requires many image reading settings (resolution and the like) to be made, it is necessary to set the light emitting element so that a light amount that is appropriate for each line period is emitted. The line period refers to a reading interval set for reading an original document line by line. When the line period is long, an image can be appropriately read even with a small light amount. On the other hand, when the line period is short, it is necessary to increase the light amount to appropriately read an image.
Therefore, to appropriately read an image, for example, when the power supply is switched on, an appropriate light amount must be set for each image reading setting, in other words, for each line period.
Patent Document 1 (Japanese Patent Application Laid-open No. 2013-73286) discloses a paper sheet recognition apparatus that detects a transporting speed of a banknote BL to be read, and according to the transporting speed of the banknote BL, corrects the light amount emitted by a light emitting element.
However, in the above-described paper sheet recognition apparatus, when correcting the light amount emitted by the light emitting element according to the transporting speed of the banknote BL, it is necessary to determine an extent to which the light amount emitted by the light emitting element is to be corrected.
For example, one approach is to, for each line period, emit a light from the light emitting element at a given light emitting amount, receive the reflected light with the light receiving element, calculate a luminance based on the received light amount, and determine the light emitting amount of the light emitting element so that the calculated luminance is within a predetermined range.
In the above-described method, if the calculated luminance is not within the predetermined range when a light is emitted by the light emitting element at a given light emitting amount in a given line period, the light emitting amount is changed to determine an appropriate light emitting amount so that the luminance is within the predetermined range.
In this manner, because the operation of determining an appropriate light emitting amount for each line period so that the luminance is within the predetermined range is performed by repeatedly performing (1) changing of the light emitting amount, (2) measurement of the received light amount, and (3) calculation of luminance, a considerable time is taken to determine an appropriate light amount for all the line periods.